The invention regards a passive mode-locked femtosecond laser having a ring resonator comprising a laser-active element, an optical output coupler and at least one mirroring element. Further the invention regards a femtosecond laser according the preamble part of claim 7 and still further a femtosecond laser according to the preamble part of claim 1. Further the invention regards a method for tuning a femtosecond laser. Also the invention regards a method of use of a femtosecond laser for generation of laser pulses with a duration below one picosecond.
With passive mode-locked femtosecond laser systems relatively high puls repetition rates in the range of several hundred MHz may be reached. These laser systems nevertheless cannot be referred to as being of high repetition rate as their repetition rates do not exceed 500 MHz. Due to extremely low pulse durations the pulses have a high peak intensity. Femtosecond lasers nowadays are successfully employed in the field of time-resolved spectroscopy, non-linear optics, multiple-photon-microscopy, micro-material engineering, optical frequency metrology and optical coherence-tomatography. Further in the future such lasers will also play a substantial role in the field of optical data communication.
Conventional passive mode-locked femtosecond laser systems rely on the use of titan doped sapphire crystals as laser-active elements. Upon optical excitation these develope a broad fluorescence spectrum in the range between 700 and 1000 nm. From this a gain profile of comparable range results, which means that Ti:Sapphire laser systems are suitable for generation of laser light in said range of wavelengths.
For generation of ultra-short laser pulses it must be observed that a laser pulse which is short in the time domain is correlated with a broad frequency spectrum. Because of this reason only laser elements with a broad gain profile are suitable for generation of ultra-short pulses.
All yet known passive mode-locked femtosecond laser systems with a solid state laser-active element (a CPM-dye laser is also passive mode-locked) rely on the concept of xe2x80x9cKerr-lens mode-lockingxe2x80x9d. This non-linear optical effect results, by self-focussing of an intensive light beam in a non-linear medium, in a temporarily gain of a single pulse in the laser-active element during its round trip in the resonator, compared to a continuous operation of the laser.
The repetition rate of such a femtosecond laser system is determined by the duration of a round trip of the pulse circulating in the resonator.
The duration of the pulse circulating in the resonator is nevertheless not able to reach the theoretical limit, which is determined by the width of the gain profile of the laser-active elements. This is caused by the phenomenon of pulse-broadening, which is experienced by the laser pulse in particular in the laser-active element during its roundtrip in the resonator. This effect is due to the so-called positive group velocity dispersion of the laser-active elements and further optical components in the resonator. The consequence is, that the various portions of wavelengths of the circulating pulse pass the laser-active element within varying time periods, whereby the laser pulse passing through the laser-active element is broadened in its time duration.
The basic approach to compensate pulse broadening which is caused by the positive group velocity dispersion of the laser-active elements and further optical components of the resonator is the use of an arrangement in the laser resonator comprising a negative group velocity dispersion which at least compensates the pulse broadening which has been caused by the laser-active element and the further optical components.
Known in prior art is for instance an arrangement of two dispersive elements, for instance prisms in a laser resonator, a so-called prism compensator.
Basics of femtosecond laser systems which rely on xe2x80x9cKerr-lens mode-lockedxe2x80x9d Ti:Sapphire lasers with prism compensators can for instance be drawn from the publication of D. E. Spence, P. N. Kean, W. Sibbet in Optics Letters 16, page 42 and following pages (1991).
Recently as an alternative to said prisms or prism compensators dielectric mirrors have been developed which provide a negative group velocity dispersion GVD. This is achieved by a suitable sequence of dielectric layers on a substrate. The basic concept can be drawn from the publication of R. Szipxc3x6cs, K. Ferencz, Ch. Spielmann, F. Krausz in Optics Letters 19, page 201 and following pages (1994).
The use of such mirrors with negative group velocity dispersion GVD in a laser resonator offers a substantial advantage, i.e. in contrast to the above mentioned prisms- or prism compensators only a non-significant prolongation of the optical path in the resonator occurs.
A femtosecond laser system, which is based on said mirrors, may for instance be drawn from the publication of H. Stingl, Ch. Spielmann, R. Szipxc3x6cs, F. Krausz in Conference on Lasers and Electro-Optics 9, 1996 OSA Technical Digest Series (O.S.A., Washington D.C., 1996) page 66 and following.
Most of the Ti:Sapphire femtosecond laser systems relying on the phenomenon of xe2x80x9cKerr-lens mode-lockingxe2x80x9d comprise a Fabry-Perot-resonator, whose markable feature is a planar end mirror and which in particular has a folded configuration. In such kind of configuration elements belonging to a pulse compression such as prisms may be allocated simply in one arm of the resonator. The total length of such a resonator amounts typically in the range of 2 meters. Therefrom typical pulse repetition rates in the range of a few megahertz of normally below 100 MHz result. Such laser systems are not labelled as having a high-repetition rate.
From the above-mentioned publication of A. Stingl et. al. for instance a passive mode-locked Ti:Sapphire femtosecond laser system is known, which relies on a Fabry-Perot-resonator and which makes use of mirrors having a negative group velocity dispersion GVD.
Further from the U.S. Pat. No. 5,383,198 a self-starting passive mode-locked femtosecond laser system is known, having a prism compressor and a ring resonator and also from the U.S. Pat. No. 5,799,025 a self-starting passive mode-locked femtosecond laser system is known, having a prism compressor and a Fabry-Perot-resonator.
Due to the respective resonator geometries none of the mentioned laser systems allows to achieve pulse repetition rates of above 500 MHz and therefore these laser systems cannot be labelled as having high repetition rates.
An alternative approach may be drawn from the publication of M. Ramaswamy and J. G. Fujimoto in Optics Letters 19, page 1756 and following pages (1994) (see also U.S. Pat. No. 5,553,093). The approach is based on a simplified resonator configuration with making use of a specific prism compressor. Instead of a conventional resonator-internal pair of prisms a prism-shaped laser crystal and a prism-shaped output coupler is used. A specific geometry of the laser resonator configurated as a Fabry-Perot-resonator and also the simplified prism compressor allows a shortening of the resonator length to about 30 cm so that a repetition rate of 1 GHz may be achieved.
Disadvantageous on this concept is, that in the outgoing beam the various in a laser pulse superimposed spectral components spread apart in a spatial direction perpendicular to the direction of the laser beam (xe2x80x9cspatial chirpxe2x80x9d), which at least complicates a practical use of such laser concept.
In fact the prism compressor being located inside the resonator enforces a minimal length of the resonator upon which it appears unlikely and even impossible that higher repetition rates than 1 GHz may be achieved.
A prism compressors causes, additionally to a negative group velocity dispersion, in some parts of the resonator a spatial split-up of the various spectral components of a laser pulse circulating in the resonator. A selection of wavelength may be established by use of a suitable, re-allocatable aperture in such area of a resonator, in which the spectral components are spatially split up. For such accomplished selection of wavelength the resonator facilitates laser activity. Thereby an ability of wavelength tuning is achieved. From the femtosecond laser system xe2x80x9cTsunamixe2x80x9d of the Spectra Physics Company, Mountain View, Calif. such kind of arrangement is known for instance.
However, due to the respective resonator geometries further with none of the mentioned laser systems it is possible to achieve pulse repetition rates exceeding 500 MHz may be achieved and simultaneously the ability of wavelength tuning. Therefore, these laser systems cannot be labelled as being of high repetition rate.
It is therefore an object of the invention to provide a passive mode locked high repetition rate femtosecond laser by which laser pulses having a duration below one picosecond can be generated and which at the same time can be operated conveniently at repetition rates above 500 MHz, in particular above 1 GHz. A laser system according to a first variant of the invention shall in particular be used to generate such laser pulses.
Furthermore such laser system shall be adapted to generate therewith continuous tuneable laser pulses within the range of the gain profile of the laser-active medium and with a duration below one picosecond and which laser system is at the same time conveniently operable at repetition rates of above 500 MHz, in particular of above 1 GHz. A laser system according to a second variant of the invention shall in particular be used to generate such laser pulses.
In such laser systems the laser beam coupled out from the laser resonator shall not suffer from a spatial spreading of a spectral component perpendicular to the direction of the laser beam.
Furthermore a required occupied area of such a laser system shall be significantly reduced compared with laser systems of prior art, in particular compared with commercial available laser systems.
Additionally an especial suitable method for tuning a high repetition rate laser system shall be provided.
To solve such object the invention in a first variant proceeds from a passive mode locked femtosecond laser according to the preamble part of claim 7.
Such a femtosecond laser comprises a laser-active element which is located between the concave surfaces of two concave mirrors. The resonator may also comprise further mirroring or other optical elements, for instance planar mirrors. A respective ring resonator comprises additionally at least one dielectric mirror, i.e. one or a number of them. A dielectric mirror has a negative group velocity dispersion GVD. The negative group velocity dispersion GVD is adapted such that for a contiguous portion of the optical spectral range which is capable of being amplified by the laser-active element, the sum of the negative group velocity dispersion GVD of the dielectric mirror (or, if so, of the number of dielectric mirrors) and the positive group velocity dispersion GVD of the laser-active element (and, if so, of further optical elements), is negative. This means             ∑      n        ⁢          GVD      n         less than   0
This is the basic assumption for a generation of femtosecond laser pulses. Further an optical output coupler is arranged in the resonator.
According to a first variant of the invention such a resonator is construed such that the optical path length in the resonator is below 60 cm, in particular below 30 cm, in particular below 15 cm. From these optical path length pulse repetition rates result which conveniently exceed 500 MHz, which in particular exceed 1 GHz and which in particular exceed 2 GHz. Furthermore the focus length of the concave mirrors, which are spatial adjacent next to the laser-active element are elected to be below 3 cm, in particular below 2 cm, in particular below or equal to 1,5 cm elected. With such a laser a high repetition rate pulsed laser operation with femtosecond pulses is achievable so that said femtosecond laser is a high repetition rate femtosecond laser.
A passive mode locked high repetition rate femtosecond laser comprising these features provides a series of substantial advantages. Making use of a ring resonator instead of a linear Fabry-Perot resonator allows a significant reduction of the resonator length, which is a basic assumption to be able to achieve high repetition rates.
Making use of dielectric mirrors with negative group velocity dispersion GVD instead of conventional prism- or grating-compressors allows additionally to reduce the length of the ring resonator. Thereby it is possible to reduce the geometric length of the resonator by more than 10 cm, which in turn is accompanied by an increase of the pulse repetition rate.
Furthermore the invention has arisen from the idea that making use of concave mirrors, which are especially arranged adjacent next to the laser-active element and whose focus length amount within the mentioned predetermined range according to the characterising part of claim 7, allows to achieve a beam diameter of a resonator mode in the laser-active element, which is comparable to the evolution of the resonator mode of a conventional linear Fabry-Perot- or also of a ring-resonator each having a significant larger geometric length. Such a small diameter of the resonator mode in the laser-active element, in particular of a beam waist in the laser-active element is a basic condition for an efficient generation of laser pulses. In particular this is a basic condition for both of the mentioned fundamental effects, which are responsible for the generation of ultra short pulses in a passive mode locked laser system. These fundamental effects are the so-called self-phase-modulation SPM and the so-called self-amplitude-modulation SAM in a non-linear medium, in particular also in a laser-active medium such as the laser-active element in a resonator. Both effects in turn are due to the so called Kerr-effect. The dependence of the index of refraction from the local light intensity is referred to as Kerr-effect.
The self-phase-modulation SPM occurring in the laser crystal is characterised by the quantity "PHgr" and is substantial for the minimal achievable pulse length xcfx84. The relation holds:   τ∞  ⁢            "LeftBracketingBar"      D      "RightBracketingBar"              Φ      ⁢              xe2x80x83            ⁢              E        p            
Therein D is the group velocity dispersion GVD summarised for all optical elements in the resonator:   D  =            ∑      n        ⁢          GVD      n      
Ep refers to the pulse energy.
The magnitude of the self-phase-modulation "PHgr" is essentially proportional to the square of the beam radius w in the laser crystal. To aim for a preferably high intensity in the laser crystal it is therefore inevitable to achieve a preferably small beam diameter of the resonator mode in the laser-active element, in particular a beam waist with preferably small diameter in the laser-active element. This specific effect is achieved according to the invention by making use of a convex mirror having a focus length adapted to the above mentioned maximal resonator length.
Thereby the further advantage is achieved, that the laser beam coupled out of the laser resonator is free from a spatial spreading of the spectral components perpendicular to the beam direction.
Also the required occupied area of the proposed laser system is significantly reduced compared to known laser systems of prior art, in particular to commercially available laser systems, which is achieved by the skilful choice of resonator geometry.
Further to solve the object, the invention in a second variant proceeds from a passive mode locked femtosecond laser according to the preamble part of claim 1. This in particular regards the generation of continuously tuneable laser pulses within the gain profile of a laser-active medium.
Such a femtosecond laser according to the second variant of the invention comprises like the first variant of the invention, a laser-active element, which is located between the concave surfaces of two concave mirrors. Additionally to the further features of the ring resonator as mentioned with the first variant (at least a dielectric mirror, two concave mirrors an output coupler), the second variant comprises a self-focussing element and a prismatic element.
According to the second variant of the invention said resonator is configured such that the optical path length in the resonator is smaller than 60 cm, advantageously smaller than 30 cm, in particular smaller than 15 cm. From these optical path lengths pulse repetition rates result, which conveniently exceed 500 MHz, in particular exceed 1 GHz, which are in particular higher than 2 GHz. Further the focus length of the concave mirrors, which are arranged adjacent next to the laser-active element, are selected to be below 3 cm, advantageously below 2 cm, in particular below or equal to 1,5 cm. Furthermore, at least the dielectric mirror or the output coupler or any further mirror is tiltable such that, due to a tilted angle, in correlation with the spatial dispersion of the prismatic elements, a wavelength is advantageously continuously tuneable for which wavelength the resonator allows laser operation.
With such a laser also a high repetition rate laser pulsed operation of femtosecond pulses is attainable so that such laser is a high repetition rate femtosecond laser. Furthermore, the proposed high repetition rate femtosecond laser is continuously tuneable. The resonator comprises a prismatic element, which functions to effectuate a spatial separation of different spectral components of the light circulating in the resonator. According to its second variant the invention arises from the idea, that in correlation with a mirror tiltable upon an axis perpendicular to the plane in which the spectral components are separated, a wavelength is selectable by said tilt such that for said wavelength the highest overall gain is supplied in the resonator. This wavelength thereon is the operating wavelength of the laser.
Although a tuneability may be achieved with most femtosecond laser systems only with some expenditure and will mostly as a general rule be achieved only to the expense of pulse duration and repetition rate (e.g. by making use of a prism compressor, which is room-expensive and supports significantly the broadening of pulses) it is nevertheless in particular advantageous to provide a wavelength-tuneable femtosecond laser. This is because such laser allows to adapt the optical wavelength within the range of the underlying gain profile to the requirements of a desired application. This has been succeeded with the femtosecond laser according to the second variant of the invention in an especial elegant and simple way. This is because the proposed continuous tuneable high repetition femtosecond laser comprises also the advantages of the laser according to the first variant of the invention. In particular the ability of high repetition, compactness, tuneability and femtosecond operation mode have been realised in combination.
Thereby it is advantageous, according to a further developed configuration of the in particular second variant of the invention, that the laser-active element also functions as a self-focussing element, i.e. the laser-active element and the self-focussing element are identical. For instance this is the case for titan sapphire lasers.
Advantageously the resonator is, in correlation with the non-linear element, configured such that pulsed laser operation allows a higher degree of energy efficiency from the laser-active element than in a continuous operation mode.
The prismatic element is advantageously configured such that an optical beam incident on a prism surface under the condition of minimal deflection, is incident in the Brewster angle with a wavelength amplifiable by the laser-active medium and also emerges under such angle from the output surface.
Optional such prismatic element may be also configured such that an optical beam incident on the prism surface with the Brewster angle with a wavelength amplifiable by the laser-active medium emerges from the output surface essentialy in a right angle. Therefore an anti-reflex layer is applied to the output surface, the anti-reflex layer being adapted for those wavelengths amplifiable by the laser-active medium. The above mentioned further developed configurations of the invention may be in particular applied to the second variant of the invention and also to the first variant of the invention.
In particular it is to be observed upon the further developed configurations of the invention according to its first variant and also to its second variant that all further parameters of the resonator are selected such that an optical stable resonator is being configured.
It has been shown that an especial advantageous laser system is available if the focus length of the concave mirrors which are spatial adjacent next to the laser-active element are elected essentially in equal way. This is enabled mainly by an essentially axial symmetric formation of the laser resonator.
In a further preferred development of the laser system according to the invention the geometric distance of the concave mirrors to each other is selected smaller than the sum of the focus lengths of the concave mirrors. By solely varying the distance d of the concave mirrors to each other while keeping the total remaining resonator geometry constant one is able to determine an interval of this distance within which a stable operation of the laser resonator is possible. Such interval is arranged essentially symmetric to a distance of the concave mirrors which distance corresponds to accurately the sum of the focus length of the concave mirrors.
It has become apparent that an especial stable operation in a passive mode locked condition is possible if the distance d of the concave mirrors to each other is selected smaller than the sum of the focus length of the concave mirrors. A positive effect on the evolution of the Kerr-effects substantial for passive mode locking results.
This effect can even be enforced if the laser-active element is arranged between the adjacent concave mirrors not in a symmetric but as defined according to the features of claim 10. If such arrangement of the laser-active element is realised a decrease of the beam diameter in the laser crystal along with an increasing instantaneous power of the pulse results. This effect is also based on the Kerr-effect and is referred to as a formation of a xe2x80x9csmooth aperturexe2x80x9d if it is in combination with a stronger focussing of the pump-laser beam as compared to the resonator mode. It also supports a stable passive mode locked pulsed operation.
In a further developed configuration of the laser system according to the invention the distance d between the concave mirrors is selected larger than the sum of the focus length of the concave mirrors. In this case the occurrence of a xe2x80x9csmooth aperturexe2x80x9d of the laser-active element may not be observed. However, instead of this a so-called xe2x80x9chard aperturexe2x80x9d may be located in the ring resonator, which for instance may be configured as a ring aperture. In particular such a hard aperture is provided at such location in the resonator on which the resonator mode has a comparingly small diameter with a comparingly large instantaneous power.
It has become apparent that a laser system according to the invention is advantageously used with a titan doped sapphire crystal as a laser-active element. Nevertheless further also the use of other laser-active elements is possible which comprise a gain spectrum broad enough to theoretically enable the generation of femtosecond laser pulses. In particular the laser-active elements of claim 12 are to be mentioned in this context.
To provide optical pumping to the laser-active element for instance an argon ion laser may be used which in particular is tuned to the maximum absorption of the laser-active element.
In an especially advantageous developed configuration of the laser system according to the invention a solid state laser system is employed as a pump laser whose wavelength spectrum is selected to be adapted to the absorption spectrum of the laser-active element. In particular the frequency doubled solid state laser systems as mentioned in claim 13 are suitable in this context. The use of a naturally low-noise solid state laser system as a pump laser effectuates in an advantageous way the fluctuation of intensity of the pulsed laser radiation generated from the inventive laser system.
Further it has become apparent that to support a stable passive mode located pulsed operation with a comparingly well degree of efficiency of the inventive laser system the degree of outcoupling T of the optical output coupler should be below 5%, advantageously below 3%, in particular below or equal to 2%. Higher degrees of outcoupling T would increase the quantum efficiency of the laser system, smaller degrees of outcoupling T would stabilise the inventive laser system in the passive mode locked pulsed operation mode. The output coupler is advantageously configured as a partial-reflecting, in particular, as a dielectric mirror.
Alternatively also other possibilities for outcoupling may be realised, for instance by outcoupling of an evanecent wave which results from an inner total reflection of a resonator mode on an interface. In such a configuration a degree of outcoupling may be varied freely within specific bounds.
In a further developed configuration of the inventive laser system all mirrors of the ring resonator have a negative group velocity dispersion GVD. Respectively, depending on the dimension of the positive group velocity dispersion GVD due to the laser-active element, also only a single mirror with negative group velocity dispersion GVD may be employed in the laser resonator. In particular also the outcoupling mirror may have a negative group velocity dispersion GVD.
In a particular advantageous developed configuration of the inventive laser system the ring resonator is construed, such that the resonator is compensated for astigmatism. This may be realised by a suitable choice of the resonator geometry, in particular of the angle of reflection of the concave mirrors. In particular a beam waist with circular cross-section in the laser-active element and/or a beam coupled out from the laser resonator having a circular cross-section may be realised in this way. It is especially advantageous for compensation of astigmatism, when the optical length of the ring resonator is selected to exceed 1 cm, advantageously to exceed 2 cm and is selected in particular above 3,5 cm. This is because at smaller wavelengths extremely high angles of reflection at the concave mirrors are to be achieved for compensation of the astigmatism of the laser-active element.
Particular practical advantages by making use of the inventive laser system result when all elements of the ring resonator are arranged mechanically on a common mounting platform. In particular a mounting of the optical components in a monolithic block is possible.
Particular advantages of the inventive laser system thus are based on its high pulse repetition rate which may extend between 500 MHz and about 10 GHz. Further advantageous is its significant reduced demand for occupied expanse, i.e. occupied space, compared to conventional laser systems. Saving of costy room on optical tables is accomplished. Finally the signal to noise ratio relating fluctuation of intensity is improved by more than a factor of 10 compared to conventional, in particular gas laser pumped laser systems. Further the advantage is achieved, that the laser beam coupled out of the resonator does not show any spreading of spectral components perpendicular to the beam direction. Even further a variant of the invention allows for the ability of continuous tuning of the inventive laser system.